1. Industrial Field of Application
The present invention relates to a semiconductor device using crystalline semiconductor, and to a method for fabricating the same.
2. Discussion of Prior Art
Thin film transistors (referred to simply hereinafter as “TFTs”) are well known as devices utilizing thin film semiconductors. The TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter. The TFTs are widely used in various types of integrated circuits, and are particularly noticed in the field of switching elements that are provided to each of the pixels of active matrix liquid crystal display devices as well as in driver elements of the peripheral circuits thereof.
Amorphous silicon films can be utilized most readily as the thin film semiconductors for TFTS. However, an amorphous silicon film has a problem that the electrical characteristics thereof are inferior. This problem can be circumvented by using a thin film of crystalline silicon. Crystalline silicon film is also denoted as, for example, polycrystalline silicon, polysilicon and microcrystalline silicon. A thin film of crystalline silicon can be prepared by first forming a thin film of amorphous silicon, and then crystallizing it by heat treatment.
The heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600° C. or higher for a duration of 10 hours or longer. Such a heat treatment has a problem that a glass substrate cannot be used. For instance, a Corning 7059 glass commonly used for the substrate of an active matrix liquid crystal display device has a glass distortion point of 593° C., and is therefore not suitable for large area substrates that are subjected to heating at a temperature of 600° C. or higher.